A4 Cabrio 2.5TDI BDG: grauer Rauch bei Kaltstart | Posts 64+

I'd also like to suggest the possibility that the glow plugs might be too short. I experienced this with my A2 3L. While it still started reasonably well in the winter, the additional glow plugs were already defective after about 150,000 km, but the noise was excruciating. The glow plugs were about 1.5 cm too short. So, even when they were still functioning, they probably had little effect on the starting process.

My Allroad 2.5 TDI AKE, on the other hand, started very reluctantly, even with only 2 functioning glow plugs. It was similar to what's shown in the video, except that in my case, the 2 cylinders with the good glow plugs ran perfectly, while the other cylinders that were reluctant to ignite took some time to actually fire.

Okay, so the car was just at the Bosch service center, which is (almost) around the corner.
There, my request to expand and inspect the combustion chamber was fulfilled, and during the inspection of cylinder four, a glow plug was found to be quite heavily coated with soot.
Whether dirt was present at the sealing point during installation, or whether the spark plug simply wasn't tightened correctly, no one knows for sure.

I now just have to wait until the engine is completely cold, and then I will report back.

Does the length or glow plug type fit?

Yes, they are correct, according to the Bosch service center. The model that is now installed, which was also used by Audi about a year ago, is the Beru GN008.

dieselschrauber wrote:

Another possibility would be a coolant temperature sensor that only provides incorrect readings in cold temperatures; I've experienced that before.

Right now, it's beautifully cold. Check the coolant temperature that the engine control unit (ECU) sees when the engine is cold, and compare it to the outside temperature.

Best regards, Rainer.

Unfortunately, I won't be able to get the car started today because I need it to get to work. However, it should work eventually, as I checked the temperature and it was already showing 1.X degrees even when the outside temperature was around -2 degrees.

Just to be sure: Has the car been modified? If an artist captured the wrong license plate number...

No, absolutely nothing has been changed on that car, it's really quite conventional .

Okay, I have good news: it was the spark plug that was causing the problem.
This morning, I started the car that was parked outside in the -4°C (25°F) temperature, and the engine started right up and sounded as it should.
There was absolutely no smoke produced.
So, it turned out that the glow plug wasn't properly seated.

So: the case is solved, thank you for your help!

Thank you for your feedback!